Low temperature method for producing amorphous boron-carbon deposits



United States Patent US. Cl. 117-46 Claim ABSTRACT OF THE DISCLOSURE Amorphous boron-carbon is deposited, at very low absolute pressure, on a substrate heated to 700-900 C., from a reactant gas mixture of boron hydride and acety lene. #Preferably, the substrate is a filament, the pressure is 5 mm. mercury, the temperature is 850 C., the boron hydride is diborane, and the reactant gas mixture also includes hydrogen.

INTRODUCTION This invention relates to a process for producing amorphous boron-carbon deposits and more particularly to a process in which amorphous boron-carbon deposits are produced at relatively low temperature.

BACKGROUND OF THE INVENTION Amorphous boron-carbon deposits, either in the boron carbide form or in non-stoichiometric mixtures of boron and carbon, have been found to have highly desirable mechanical properties. In particular, these materials are known for their extremely high tensile strength and modulus of elasticity even at very high temperatures. It should be noted that while this material is referred to herein as amorphous, this term is not intended to signify the complete absence of a crystalline structure in the deposit but instead indicates only that no crystalline structure is discernible by the presently available techniques, such as X-ray diffraction, etc.

Despite the highly desirable mechanical properties of amorphous boron-carbon deposits, utilization of these materials has been limited generally by the fact that prior art processes for producing such materials have required process temperatures which would destroy or cause degradation of many substrate materials. Further, these processes have been relatively slow, and have not been capable of consistently producing high quality deposits.

For example, the most common method for producing amorphous boron-carbon deposits heretofore has been to heat the substrate to temperatures above 1100 C. in the presence of boron trichloride, methane, and hydrogen. Substrates which undergo undesirable physical or chemical changes in this temperature range cannot be used in this process.

OBJECTS It is an object of the present invention therefore to provide a method for producing amorphous boron-carbon deposits at a relatively low temperature.

Another object of this invention is to provide a practical low temperature process for producing amorphous boron-carbon deposits in a form which can be utilized as a structural material reinforcement.

BRIEF SUMMARY OF THE INVENTION These and other objects are met, in accordance with the present invention, by a process which comprises, briefly, heating a substrate material to a temperature in the range from 700-900 C. at a very low pressure and contacting the substrate material with a gaseous mixture 3,537,877. Patented Nov. 3, 1970 of acetylene and a boron hydride. In the preferred form of the present invention a filamentary substrate is contacted with a mixture of acetylene, diborane, and hydrogen at a temperature of about 850 C. and a pressure of about 5 millimeters mercury.

DETAILED DESCRIPTION OF THE INVENTION While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the present invention, this invention may be better understood from the following description.

With regard generally to the boron hydrides which constitute the source of boron in the present invention, a variety of volatile boron hydrides may be useful although diborane is preferred because of its relative volatility, availability and low cost. These materials are relatively stable at ordinary temperatures and apparently undergo controlled decomposition at the reaction temperature and pressure of the present process. Although the specific mode of decomposition is unknown, it is likely that numerous intermediate products, including higher molecular weight boron hydrides, are formed as the decomposition proceeds. This further suggests that other boron hydrides may be used in lieu of, or in addition to, the better known and more readily available diborane.

While amorphous boron-carbon deposits may be produced at low pressure by decomposition of a mixture consisting only of a boron hydride and acetylene, hydrogen is usually included in the reactant gas mixture. The role of the hydrogen is two-fold. First, it aids in the reduction to boron carbide, and second, it moderates deleterious side reactions.

The effectiveness of the present invention to produce boron-carbon deposits in the temperature range below 1000 C. is an important feature of the process taught herein. It permits the production of amorphous boroncarbon deposits on many substrates which would undergo undesirable physical or chemical changes at temperatures above 1000 C. The latter category includes metallic substrates, which may undergo crystalline changes or exhibit a loss in ductility if they are heated above 1000 C., and also includes substrates such as silica and glass which would deteriorate at temperatures above 1000 C. The latter are particularly desirable substrates for use in producing low density boron-carbon filaments. Such filaments, produced, in accordance with the present invention, by the deposition of amorphous boron-carbon on a filamentary silica or glass substrate, have excellent potential as the reinforcing constituent of composite structural materials.

The quality of these products has been determined by visual observation as well as by X-ray and microscopic analysis which showed the deposits to be relatively smooth, amorphous, and free of imperfections.

In one demonstration of the present invention, a tungsten filament, 1 mill in diameter, was mounted axially in a horizontally disposed cylindrical reactor 2 inches long and inches in internal diameter. This reactor included electrical contacts for resistively heating the filament in the reactor and means for maintaining a high vacuum and a low concentration of contaminants in the reactor. Other features of the reactor included inlets and outlets for streams of reactant gases flowing generally perpendic ularly to the reactor axis and a reactant gas diverter, comprising a slotted barrier between the gas inlets and the filament in the reactor, for directing the reactant gases into the space immediately surrounding the filament. With reactor pressure reduced to 5 millimeters mercury absolute, the temperature of the substrate was raised to about 850 C. by resistive heating. A mixture of hydrogen, flowing at 20 cubic centimeters per minute, diborane 3 flowing at 120 cubic centimeters per minute, and acetylene flowing at 30 cubic centimeters per minute was passed through the reactor. This resulted in production of an amorphous boron-carbon deposit at a rate, in diameter growth, of about 25 mils per hour.

The filament produced in the above example was hard and, under X-ray analysis, devoid of any discernible crystal structure. In order to prove that the deposit was not pure boron, an attempt was made to etch it with 50% hydrogen peroxide at 100 C. No etching occurred, which indicated that the deposit was not simply boron. Since carbon would not be expected to be deposited at the low temperature of this experiment, it is also unlikely that the deposit produced was simply carbon. Therefore, it was concluded that the deposit produced in the experiment was amorphous boron-carbon. A portion of the deposit may have been boron carbide but this could not be proven.

In one possible adaptation of the present invention, an indefinite length of amorphous boron-carbon-coated filament may be produced by passing a substrate filament of indefinite length through a reactor of the type described in the above example. This filament would be continuously heated and contacted with the gaseous reactant mixture as it passes through the reactor.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A process for producing an amorphous boron-car- References Cited UNITED STATES PATENTS 2,671,735 3/1954 Grisdale et al. 2,810,365 10/1957 Keser. 2,853,969 9/ 1958 Drewett. 3,125,428 3/1964 Maczka. 3,365,330 1/1968 Hough 1l7-201 OTHER REFERENCES Powell et al., Vapor Plating, 1955, pp. 3, 4 and 8 to 11, relied upon.

ALFRED L. LEAVITT, Primary Examiner J. H. NEWSOME, Assistant Examiner US. Cl. X.R. 1l7-106 

